1. Field of the Invention
The invention relates generally to the field of electronic circuits, and more specifically to a new and improved ramp generator circuit which is particularly useful as a circuit for controlling electron-beam deflection in a cathode ray tube for a video display monitor.
2. Background
Video display devices include a cathode ray tube for displaying information in the form of visual images on a generally planar screen. In a cathode ray tube, an electron beam is directed from an electron gun toward the screen. Where the beam impinges on the screen, a phosphor coating on the screen fluoresces, resulting in the generation of light at that point which can be observed by a user. The electron gun is pointed generally towards the center of the screen, but the beam is moved horizontally and vertically to permit the beam to scan the surface of the screen by means of magnetic fields generated in coils adjacent the tube between the gun and the screen. As the beam scans, the amplitude of the beam can be varied to result in the generation of light and dark patterns over the face of the screen. The patterns may be, for example, in the shape of alphanumeric characters, that is, text, or graphic images, or some combination of both, depending on the type of information to be displayed.
Two general paradigms are implemented in controlling scanning of the electron beam to generate images in visual display devices. In one, which is commonly implemented in computer video displays and television sets, the electron beam is scanned over the screen in a raster pattern, that is, the beam is directed in a series of horizontal lines across the screen from the top of the screen to the bottom. Horizontal deflection, which enables each line to be generated, and vertical deflection, which enables the series of lines to be generated from the top of the screen to the bottom, are controlled by circuits which are designed to generate deflection signals which vary at a predetermined rate for the display device to generate the raster pattern. As the beam is scanned over the screen, its amplitude is varied to generate the visual images; if the amplitude does not vary, the entire screen will have the same brightness.
In the other paradigm, which is typically used in oscilloscopes, the electron beam scans across the screen, not in a series of lines, but instead in a single line enabled by a horizontal deflection circuit. Typically, vertical deflection is caused by a signal which is input to the oscilloscope by a user. In that case, the user may analyze the input signal as a function of time, which is the time required for the electron beam to scan across the screen. In addition, in a typical oscilloscope, instead of using the oscilloscope's horizontal deflection circuit, the user may input another signal to cause the horizontal deflection. This permits the user to analyze the input signals in relation to each other.
A typical horizontal deflection circuit essentially implements a resonant circuit which has, at different times, two resonant frequencies. The circuit includes a deflection coil which generates a magnetic field that, in turn, controls the deflection of the electron beam. Essentially, when a bipolar transistor is switched from an on condition to an off condition, the resonant circuit operates at a high frequency during which the current through the coil changes relatively quickly, resulting in a relatively fast retrace of the beam from the right edge of the screen to the left edge. During retrace, a very high flyback voltage is developed across the transistor.
When the beam has fully retraced to the left edge of the screen, the resonant circuit changes to a low resonant frequency. When that occurs, the current through the coil changes relatively slowly, resulting in a relatively slow scan of the beam from the left edge of the screen to the right edge. During the scan, the bipolar transistor turns back on again and essentially saturates as relatively large currents flow therethrough.
While bipolar transistors can withstand the high flyback voltages that are developed during retrace, they also have a high minority carrier storage in the collector-base region during saturation. This tends to limit the speed with which the transistor may be switched off, which, in turn, serves to limit the frequency at which the deflection circuit may operate to deflect the electron beam. As a result, while these deflection circuits are satisfactory for normal broadcast television and small, low-resolution video monitors used in computers, they are not satisfactory for use in high resolution monitors or monitors with relatively large screen images.
More recently, a deflection circuit has been developed in which a plurality of parallel-connected metal-oxide semiconductor field effect transistors (MOSFETs) have been substituted for the bipolar transistor. See, for example, K. Ando, A Flicker-Free 2448.times.2048 Dots Color CRT Display, SID 85 Digest, pages 456 through 460. While a MOSFET generally has almost no carrier storage, and thus could be used in a higher-frequency deflection circuits used in high resolution video monitors, normal MOSFETs are typically unable to withstand the high flyback voltages that are developed in such circuits. Thus, the circuit described in the Ando article requires the use of a specially-developed high voltage MOSFET. In addition, the connection of the MOSFETs in parallel tends to increase their effective total capacitance, which also limits the deflection frequency at which the circuit can operate.